Alternating current motor



Patented Nov. 10, 1936 UNITED STATES PATENT OFFICE ALTERNATING CURRENT. .MOTOB Austin S. Norcross, Newton, Mass. Application August 19, 1935, Serial No. 36,789

8Claims. (01.172-277) This invention relates to electric motors for operation on alternating current, and the object is to provide a motor of this type operable at different speeds. The example of the invention herein described may be characterized as. an adjustable speed, single-phase commutator motor having shunt speed characteristics. has a wide range of speed and its speed is little affected by variations in load. High torque, a

10 good power factor and good emciency are attained in operation and starting conditions are favorable.

The principles of my invention may conveniently be understood by reference to the following description of illustrative embodiments thereof, as shown by way of example in the accompanying drawing, wherein:

Fig. 1 is a circuitdiagram of the motor;

Fig. 2 is a similar diagram of a modified form;

00 and Fig. 3 is a vector diagram indicating the time phase relations of the fluxes, voltages and currents existing in the motor under running condione.

Referring to Fig. 1, I have there shown dia grammatically a motor with stator windings embodying a field coil M, which for convenience may a be denominated the main coil or winding, and is energized as indicated directly from a suitable source of supply of alternating current. A second winding Q, connected in parallel to the main winding M, is located ninety electrical degrees aparttherefrom. In other words, the coils M and Q are in space quadrature and for convenience .I-

will refer to the winding Q as the quadrature winding. i-lerein I have shown a capacitor C in the circuit of the winding Q, this being illustrative of suitable means providing for a differential in time phase between the windings M and Q, the

differential being substantially less than 90 as hereinafter more fully explained. The voltage supplied to the coil Q is adapted to'be adjustably varied in magnitude (but not in phase) and I have hereiniilustrated an auto-transformer T as interposed between the source of current supply and the winding. I

The armature A rotates within the stator and is provided with a commutator having brushes B which short circuit the same substantially on the axis of the main winding M. In the drawing they are shown on the axis, although they might in some instances be slightly displaced from the axial line for purposes of regulation, as will be apparent to those skilled in the art,

The motor- Having thus didactically described the disposition of the various parts illustrated in Fig. 1, the electrical and mechanical eflect resulting therefrom maybe briefly summarized as follows: The reaction between the current flowing in the ar- 5 mature, which is the result of a voltage made up of that induced by transformer action from winding M and a speed voltage due to the flux from Q; and the flux I q, set up by the quadrature winding Q results in a torque which rotates the arma- 1o ture, the value of the torque being determined b the fundamental equation:

wherein T is the torque, K is a constant of pro- 15 portionality and 0m is the angle between the armature current, I, and the quadrature field flux Do. The motor will operate at a speed which in accordance 'with the equation:

rature field flux, a which when added to the g transformer voltage, Em'r (induced in the armature from the main field flux) will produce a resultant armature voltage, Ea, which will in turn produce an armature current, II, of correct phase and magnitude to satisfy Equation (1). voltage Eqs to satisfy this relation may be obtained at various speeds by varying the magnitude of voltage Vq+o supplied to the quadrature winding and capacitor and consequently the value of no, the flux in the quadrature field. as

To obtain a practically useful motor, however, with adequate torque and other iavorabie chanacteristics, the angle or should be such that its cosine as a factor in Equation 1) is nearly unity, a result obtained when it approximates either 4.0 zero or a straight angle", or 180. The algebraic sign of the cosine affects the direction of rotation. This I effect by providing a differential in time phase between main-winding M and quadrature winding Q (as by suitablyflxing the magnitude 45 of capacitor C) substantially different from as above stated. In practice I have found for such designs of motor as I have used a quadrature voltage leading the main field voltage by about seventy degrees to give excellent results, which 50 figure I give by way of example, and without limitation thereto. The condition may be perhaps more fundamentally expressed by saying that the differential is such as to provide a resultant voltage due to the transformer and speed 55 The 0 voltages of the armature. which resultant voltage leads the transformer voltage.

The matter may be more fully understood by reference to the vector diagram, Fig. 3. This-is a time diagram showing only relative time phase. the transformer voltage, EMT, induced in the armature from the main field fiux, 4m, being arbitrarily taken along the X axis. The drawing is further a diagram in that it is not to scale, the length of the vectors not accurately representing magnitudes, and in that the angular positions of the vectors while generally exhibiting their relation have been somewhat arbitrarily drawn to promote clearness and an open diagram. All

vectors are supposed to rotate in a counter-clock wise direction, as indicated by the arrow, at a speed which is determined by the supply frequency.

Let me assume a main field voltage Vu as shown. As this is the voltage actually applied to the main field, the voltage induced in the main field will be less than Vu by the resistance and leakage reactance drops in the main field, IuRu and ImXu, respectively. In addition to inducing the voltage Eur in the main winding, the flux hr induces a voltage Em in the armature.

Considering now the quadrature field, the voltage VQ+C applied to its circuit has two components, one, Vc, across the capacitor, and the other, Va, across the quadrature winding. In accordance with my invention, the magnitude of the capacitor C is so chosen in connection with the other constants of the circuit as to give a quadrature field current, IQ, which lags Emby more than or, as earlier stated herein, Vq leads Vu by substantially less than ninety degrees whereby Iq, which lags its voltage by somewhat less than ninety degrees depending on the design of the motor, will lag Eu'r by somewhat more than 180'. As the current through a capacitor leads its voltage by ninety degrees, the capacitor voltage Vc will take the position shown. Likewise the inductance of the quadrature field causes the current to lag its voltage by somewhat less than ninety degrees, so that the actual quadrature field voltage'is givenby the position of Vq.

The quadrature field current Iq produces a flux l which may be assumed to be in phase with it. With the armature rotating in the fiux do there is generated in the armature a speed voltage, Eqs, in phase with the flux. I now have in the armature two voltages, a transformer voltage, Em, and a speed voltage. Eqs. These two voltages combine to give a resultant voltage, Ea, which because of the position of the latter relative to the former leads Em. The resulant voltage Ea causes the armature current I. to flow to supply torque by reaction with the quadrature field flux to. The circuit I. will adjust itself to satisfy the conditions expressed by Equation (1).

If I consider the torque it will be understood that, since the magnitude of the capacitor has caused Vc to lead VM by substantially less than ninety degrees and 4):) therefore to lag Eu'r by more than 180, En will lead Emand therefore the armature current I., which lags En by a definite amount depending on the design of the motor, will lag Emby a relatively small angle, making the angle 01 differ by a small amount from 180 and the torque high, whereas if Vq led Vm by substantially ninety degrees in, lagging VQ by less than a-quadrant, would lag Emby less than 180", and the voltage E s depending on Do, would give a resultant voltage Ea lagging Eur. The armature current would then lag Eur by a large angle and angle Or would vary by a large amount from a straight angle (180). its cosine would be small and the torque so low that the motor would be impractical.

Considering the matter of adjusting speed, it should be clear from Fig. 3 that this can be accomplished by adjusting vq+c and consequently on. The motor will change speed as s is adjusted in order to generate substantially the same Eqs (see Equation 2) which is necessary in order that E g and EMT produce a voltage En which will cause an armature current of the necessary phase and magnitude to satisfy Equation 1.

It will be obvious to those skilled in the art that adjusting V); will regulate Em and therefore also adjust speed.

In certain designs of motor, I have found that a greater speed range is attained and operating conditions improved by providing means for adjusting the voltage applied to the main field winding M incorrelation with the adjustment made in the voltage applied to the quadrate field winding Q. A motor arranged for such adjustment is diagrammed in Fig. 2.

Referring again to Fig. 3, the current L fiowing in the armature is neutralized by a current I. flowing in the stator. There is also flowing in the stator a magnetlzingand core loss current, In, which when added vectorially to L gives the resultant stator current In. The stator current In is the actual current flowing in the stator. The line current I1. is the vector sum of the main field current In and the quadrature field current Io.

A particular motor constructed in accordance with the principles of the invention has exhibited high efilclency, good power factor and high torque. The motor starts easily under load. The

change of speed due to changes in load is relatively small. A speed variation of between 800 and 2400 R. P. M. has been attained while maintaining good operating conditions by varying voltage to the quadrature field only. By varying both the main and quadrature field voltages, a much larger speed range has been obtained. Commutation under normal operating conditions is decidedly better than that of an ordinary series alternating-current motor and remains satisfactory over a wide range of speeds.

It will be clear to those skilled in the art that the direction of rotation of the motor may be reversed by suitably changing the leads. In that case, the disposition of vectors would of course be substantially different in detail from Fig. 3, but subject to similar analysis.

For convenience, I append a list of the symbols used in this specification in referring to the various electric forces considered.

Vx.=supply voltage Va -voltage impressed upon main winding winding and series capacitor Vq+c=voltage impressed upon quadrature Vc=capacitor voltage Vq=quadrature field voltage IuRu=resistance drop in main field winding IuXu=leakage reactance drop in main field winding Eu'r=induced voltage in main field winding Em-=transformer voltage induced in armature from main field fiux I =quadrature field current lagging Vq by the impedance angle of quadrature field T=YK @QII. COS 9T (1) Eqs=K in" (2) EMT=K dmf (3) Where T=torque n=speed multiplied by number of pairs of poles f= frequency Power input =V1.Ir. cos 0 Power factor=cos 0 I am aware that the invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof, and I therefore desire the present embodiment to be considered in all respects as illustrative and not restrictive; reference being had to the appended claims rather than to the foregoing description to indicate the scope of the invention.

I claim: 1 Y

1. An alternating current motor comprising a field winding, a rotor having a commutator and brushes disposed to short circuit the rotor substantially along the axis of said field winding, a quadrature winding spaced ninety electrical degrees from the field winding connected in parallel thereto and having'in its circuit a capacitor of such a magnitude as to provide in the rotor a voltage resultant of the transformer and speed voltages, which resultant voltage leads the transformer-voltage, and a voltage regulator in said circuit whereby to provide for varying the speed of the motor.

2. An alternating current motor comprising a field'winding, a rotor having a commutator and brushes disposed to short circuit the rotor substantially along the axis of said field winding, a

quadrature winding spaced ninety electrical degrees from the field winding connected in parallel thereto and having in its circuit a capacitor of such a magnitude as to provide a fiux in the quadrature field lagging the armature transformer voltage by somewhat more than 180, and a voltage reguiator in said circuit whereby to provide for varying the speed of .the motor.

3. An alternating current motor "comprising a field winding, a rotor having a commutator and brushes disposed to short circuit the rotor substantially along the axis of said field winding, 9.

quadrature winding spaced ninety electrical degrees from thefield winding connected in parallel thereto, dephasing means in the circuit of said latter winding comprising a capacitor of .such magnitude as to dephase the circuit of the quadrature winding relatively to the main winding by an amount substantially differing from ninety degrees to produce an angle having. a large cosine between the armature current and the quadrature field fiux and a voltage regulator in said circuit whereby to provide for varying the speed of the motor.

4. An alternating current motor comprising a .field winding, a rotor having a commutator and brushes disposed to short circuit the rotor substantially along the axis of said field winding, a quadrature winding spaced ninety electrical degrees from the field winding connected in parallel thereto, dephasing means in the circuit of the latter winding comprising a capacitor of such a magnitude as to cause the voltage of the quadrature winding to lead the voltage of the main field by an angle substantially less than ninety degrees and to cause the current therein lagging the voltage by the impedance angle of the quadratefield to lag the armature transformer voltage by more than 180, and a voltage regulator in said circuit whereby to provide for varying the speed of the motor.

5. A motor as defined in claim 1, wherein means are provided for. independently varying the voltage applied to the main winding.

6. A motor as defined in claim 2, wherein means are provided for independently varying the voltage applied to the main winding.

7. A motor as defined in claim 3, wherein means are provided for independently varying the voltage applied to the main winding.

AUSTIN S. NORCROSS.

CERTIFlCATE OF CORRECTION.

Patent No. 2,060,106. November 1c, 1936.

AUSTIN S. NORCROSS.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 2, first column, line 19, for "me" read us; lines 51 and 62, for "I" road we; line 59, for "circuit read current; and second column, line 65, in the equation, strike out the words "winding and series capacitor" and insert the same after "quadrature", same column, line 64; and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 26th day of January, A. D, 1937 Henry Van Arsdale (Seal Acting Commissioner of Patents. 

